In order to protect delicate components in some electronic circuits, current limiting devices are employed to prevent the flow of current to the components from exceeding some predetermined damaging value. These devices operate to hold the current to the components below a harmful value in the event of some circuit malfunction or abnormal situation.
It is sometimes desirable to be able to change one delicate component for another having a different damaging current limit. For example, a circuit for supplying a 120 ohm strain gauge might normally operate at about 42 milliamps of current at an operating voltage of about 5 volts, and might permit 50 milliamps of current to flow to the strain gauge without damage to the gauge. However, if a 350 ohm strain gauge which normally operated at about 29 milliamps of current at an operating voltage of about 10.00 volts were substituted in the same circuit, the gauge might be damaged or destroyed by a current flow of 50 milliamps. A prior art current limiting device could be used, to hold the 50 milliamps maximum current value in the event of a malfunction, which would protect the 120 ohm strain gauge. However, the same current limiting device would permit damage to the alternative 350 ohm strain gauge. Of course, a different current limiting device could be substituted for each load change, but this is costly and undesirable. Accordingly, prior art current limiting circuits are not desirable when used in connection with interchangeable load components.
The present invention provides a current limiting circuit which will prevent damaging current from flowing to delicate devices such as strain guages and still permit interchanging of such devices while, at the same time, providing a very accurate source of voltage for these delicate components.